Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Magma Extrusion: Field Relations of Volcanic Rock Bodies

247

(a)

10.7 Pahoehoe lava ﬂows. (a) Corded glassy pahoehoe tongues 1 m or so in width formed during the 1974 eruption of Mauna Ulu, Hawaii.

An active lava fountain playing in the left background over the vent feeds actively moving incandescent lava ﬂow tongues that are some-

what lighter colored in this photograph. (U.S. Geological Survey photograph courtesy of Robin T. Halcomb.) (b) Schematic serial sec-

tions through a tube-fed pahoehoe ﬂow showing major upstream lava tube and multiple smaller downstream distributary tubes. (Repro-

duced by permission from Rowland SK, Walker GPL. Pahoehoe and aa in Hawaii: Volumetric ﬂow rate controls the lava structure. Bull.

Flow direction

(b)

ﬂow can be several meters in diameter and many kilo-

meters long. If, as commonly happens, the lava drains

downslope from beneath the crusted skin, the tube

becomes an open cavity; these characterize exposed

subvertical sections through the interiors of solidiﬁed

pahoehoe ﬂows (Figure 10.7b). On gentle slopes, elon-

gate pressure ridges and more equidimensional, dome-

like tumuli are elevated segments of pahoehoe ﬂows in-

ﬂated by more lava input or stranded highs between

drained segments. Crestal clefts or tension gashes are

commonplace.

Though commonly developed on land, pahoehoe

ﬂows can also form subaqueously.

Aa and Block Flows. Aa ﬂows are thicker than pahoe-

hoe and have exceedìngly rough, treacherous surfaces

of irregular, clinkerlike scoriaceous fragments (Figure

10.8). A complementary but thinner rubble layer lies at

the base. Flow advance is faster than pahoehoe so that

the tensile strength of the cool rigid crust is overcome

by the applied stress, causing autoclastic breakup. Heat

loss is also higher from the discontinuously exposed

incandescent core, causing increased downslope crys-

tallinity in the groundmass (Polacci et al., 1999), which

is another striking contrast from glassy pahoehoe. In-

creasing crystallinity produces more irregularly shaped

vesicles than the smooth-walled subspherical vesicles

of more glassy pahoehoe. Non-Newtonian rheology in

more crystalline lava may be manifested in plug ﬂow

(Figure 8.13) and lava levées along channel margins

(Figure 10.2).

Block ﬂows resemble aa but have a mantle of more

regularly shaped polyhedral chunks rather than jagged,

highly vesicular, scoriaceous clinker. Block ﬂows range

from basaltic to highly silicic obsidian.

The contrasting morphological characteristics, as

well as observed transitions from pahoehoe to aa, never

the reverse, suggest that greater apparent viscosity pro-

motes development of aa (Wentworth and Macdonald,

1953). Any downslope loss of heat and gas (causing in-

creasing crystallization) allows near-vent pahoehoe to

become aa farther downstream. Degassing and increas-

ing nucleation and crystallization are promoted by

stirring; consequently lava ﬂows plunging over steep

escarpments and lava produced by more vigorous

fountaining may change from pahoehoe to aa. More

viscous aa that is ﬂowing faster with greater strain rate

(Peterson and Tilling, 1980) results in fragmentation of

the surface. Calm eruption and slow advance result in

pahoehoe.

248 Igneous and Metamorphic Petrology

10.8 Toe of aa lava ﬂow in the Black Rock Desert monogenetic basalt ﬁeld, west central Utah.

Pillow Lavas. Pillow lavas are usually of low-viscosity

basaltic magma formed where it comes into contact

with water or water-saturated sediment, even in shal-

low intrusive situations (Walker, 1992). Their most

widespread occurrence is on the seaﬂoor where they

have developed by extrusion along spreading ridges

and on seamounts. Although having the appearance in

most exposures, such as roadcuts (Figure 10.9a), of a

pile of discrete, independent ellipsoids of pillow shape

and size, submarine pillow lavas in some outcrops and

especially those viewed on the seaﬂoor in three dimen-

sions consist of a tangled mass of elongate, grooved, in-

terconnected ﬂow lobes that are circular or elliptical in

cross section (Figure 10.9b). Flattening of the still hot,

not quite rigid pillows produces convex upward tops

and cusped bottoms that ﬁll openings between under-

lying pillows. Such forms are useful indicators for the

ﬁeld geologist of stratigraphic “right-side-up” direction

in deposits that have been tectonically tilted. Pillows

may resemble pahoehoe toes in cross section; they can

be distinguished by the lack of open ellipsoidal internal

tubes and the presence of fewer vesicles and radial con-

traction cracks. Pillows typically have a concentrically

zoned fabric (Figure 7.3) reﬂecting decreasing inward

rate of cooling. They may also be progressively en-

crusted with Mn-Fe oxides as the glassy envelope

“weathers” subaqueously to palagonite (Section 7.1.1).

Shattering of the hot glassy rinds of pillows in water

creates vitroclasts, discussed later.

Moore (1975) observed submarine pillows forming

off the 20°-sloping coast of Hawaii in tens of meters of

water. Toothpastelike protrusions of fresh lava squeeze

out of trapdoorlike openings in an upslope pillow; the

newly protruded pillow tongue may then detach and

roll away or, if it remains connected, may bud a new

protrusion. Pillow formation on ﬂatter slopes has not

been observed.

Smooth-surfaced sheet lava ﬂows and lava lakes

along oceanic rifts are apparently formed in deep water

by more rapid extrusion rates than those that create

pillow lavas.

10.2.2 Columnar Joints

All rocks are fractured, mostly because of tectonic

forces. However, most tabular bodies of magmatic

rock, especially aa lava ﬂows and thin sheet intrusions

of basalt, have uniformly spaced columnar joints

formed by shrinkage during cooling (Figure 10.10).

Thermal contraction creates tensile stresses that ex-

ceed the brittle strength of the rigid magmatic body.

Resulting extensional cracks nucleate at more or less

equidistant points on the upper and lower margins of

a uniformly cooling tabular body. At each of these nu-

cleation points, a randomly oriented planar crack, or

perhaps a three-prong crack, forms and propagates

away from its point of origin along the essentially

isothermal surface of the ﬂow until it intersects an ex-

tensional crack propagating from a neighboring nucle-

ation point. The network of cracks so formed consists

of rather regularly sized polygons with four, ﬁve, six, or

seven sides that resemble desiccation cracks in a thin

layer of drying, shrinking mud. As cooling and con-

traction advance into the tabular body, the polygonal

cracks likewise propagate inward, forming mostly

hexagonal joint columns; these represent the “least-

work” conﬁguration of thermally induced tensile-stress

fractures in the magma body.

Joint columns have more than aesthetic appeal. Be-

cause they develop perpendicular to isothermal cooling

surfaces parallel to the margins of a cooling tabular

body, the conﬁguration of the body, though its deﬁning

margins are now possibly missing as a result of erosion,

Magma Extrusion: Field Relations of Volcanic Rock Bodies

249

10.9 Basaltic pillow lavas. (a) Roadcut at Nicasio Dam in the

Mesozoic Franciscan Complex north of San Francisco. (Pho-

tograph courtesy of Mary Hill.) (b) Submarine pillows at a wa-

ter depth of about 2 km along the Puna Ridge east of the is-

land of Hawaii. (Photograph courtesy of Hank Chezar and

D. J. Fornari.)

(a)

(b)

can be inferred (Figure 8.19c). The pattern of jointing

is useful in delineating individually emplaced cooling

units in a succession of volcanic deposits.

10.2.3 Subaerial Lava Accumulations

On land, the three most important types of basaltic

lava-built accumulations are, in terms of increasing vol-

ume, small basalt ﬁelds, shield volcanoes, and plateau-

forming ﬂoods.

Basalt ﬁelds form on ﬂanks of larger composite and

shield volcanoes, within large calderas, and in other

continental areas. Vents may be localized along exten-

sional faults. Generally small (1 km

), monogenetic

extrusions produce simple tonguelike lava ﬂows and

associated cinder cones (Figures 10.3 and 10.4). Where

rising basalt magma encounters water-saturated sedi-

ment or surface bodies of water (lakes), hydromag-

matic explosions produce tuff rings and maars (dis-

cussed later). In some places, a few lava ﬂows may be

superposed but no thick pile is produced as in basalt

plateaus. The activity of a ﬁeld may last a few millions

of years and create hundreds of volcanic ediﬁces.

Shield volcanoes are built by innumerable extru-

sions of low-viscosity basaltic lava ﬂows from a central

250 Igneous and Metamorphic Petrology

10.10 Columnar joints. (a) Schematic view of joint columns oriented

perpendicular to an isothermal surface in a cooling magma

body. See also Figure 8.19c. (b) Multitiered columnar joints in

a remnant of a 100-m-thick, ponded basalt lava ﬂow now ex-

posed just above the Colorado River in the western Grand

Canyon, Arizona. Abruptly overlying a basal “colonnade” of

thick columns is the “entablature” of thinner columns.

Though seemingly haphazard, two radially oriented arrays can

be discerned on the left in the entablature. Many lava ﬂows

ﬁnd their way into drainage channels and are subsequently

overtopped by water, which penetrates down into the frac-

tured cooling lava ﬂow. Margins of the radial arrays may de-

lineate where the water entered, locally depressing isothermal

surfaces. (Photograph courtesy of W.K. Hamblin.)

(b)

10.11 The island of Hawaii is built of seven coalescing shield volca-

noes. See also Figure 9.8. Four of these—Hualalai, Mauna

Loa, Kilauea, and Loihi—are active. Loihi seamount south of

Kilauea has not yet emerged above sea level. Extinct

Mahukona northwest of Hualalai has submerged isostatically

(Moore and Clague, 1992). Each shield grew over about 0.6

My. (a) Aa and pahoehoe lava ﬂows, such as the historic ﬂows

of Mauna Loa (patterned and labeled), build the subaerial

parts of the shields. These ﬂows are extruded from ﬂank ﬁs-

sure (rift) systems and from a summit central vent complex.

Deeply eroded older shield volcanoes on older islands reveal

extensive feeder-dike swarms marking the ﬁssure systems.

Topographic contours are thousands of feet above sea level.

(Redrawn from Stearns, 1966.) (b) Offshore bathymetry

(depth contours in km) and major submarine slumps (dark

shade), and debris avalanches (light shade). Double lines

through subaerial shields are rift zones through which most of

basaltic lava is extruded. AA line is line of cross section in (c).

east rift zone of Kilauea shield volcano showing subaerial lava

ﬂows (subhorizontal lines), hydroclastic debris (dashed lines),

pillow lava (ellipses), sheeted feeder dikes (vertical lines), gab-

bro (dotted pattern), and magma (black). Dark shaded in

lower right is mantle. (Redrawn from Moore et al., 1994.)

KOHALA

MAUNA KEA

HUALALAI

MAUNA

LOA

KILAUEA

0mi10

0km16

1868

1877

1907

1916

1926

1919

1950

1832?

1950

1940

1926

1940

1851

1949

1942

1935

1855

1935

1942

1852

1880

1843

1750?

1859

(a)

vent complex and locally one or more ﬁssure systems

radially disposed from it; the resulting ediﬁce shape re-

sembles a warrior’s shield (Figures 9.8, 10.6, and

10.11). Small shields, as in Iceland and many continen-

tal areas, have diameters of a few kilometers. Shield

volcanoes ﬁnd their greatest development in the

Hawaiian Islands. These gigantic ediﬁces grew from

the seaﬂoor by submarine extrusions (discussed later)

for tens to hundreds of thousands of years before even-

tually becoming subaerial. They are the largest volca-

noes on Earth, have diameters of over 100 km, and rise

as much as 10 km above their base on the seaﬂoor.

(Mount Everest is only 8.85 km above sea level!) So

great is their weight that the oceanic lithosphere is

ﬂexed downward, resulting in greater ocean depths im-

mediately around the isostatically subsiding island

mass. The largest island, Hawaii (Figure 10.11), has

grown over the past 0.6 My and is composed of seven

Magma Extrusion: Field Relations of Volcanic Rock Bodies

251

(b)

MAUI

156°

155°

0 km 100

mi 50

MAHUKONA

KOHALA

HUALALAI

MAUNA

KEA

MAUNA

LOA

KILAUEA

LOIHI

20°

19°

(c)

−5

−10

−15

Rift

zone

S.L.

806040

200

Oceanic crust

10.11 (Continued ).

coalesced shield volcanoes, the oldest of which is sub-

merged to the northwest; the youngest, to the south-

east, has not emerged above sea level. Gravitational in-

stability of the ﬂanks of the compound shield ediﬁce

causes recurrent landslides (Figure 10.11b, c). These

are more or less coherent slumps of a ﬂank sector

whose slope is 3° as well as fast-moving chaotic de-

bris avalanches on gentler slopes, some of which have

gigantic volumes on the order of 5000 km

—the largest

avalanches known on Earth.

Plateau-forming, ﬁssure-fed ﬂood basalts are the

most voluminous subaerial lava extrusions of any com-

position known on Earth. Many continental plateaus

occur around the globe, including the Jurassic Karroo-

Ferrar in southern Africa and Antarctica, Paraná-

Etendeka in South America and southwestern Africa,

and the late Cretaceous-early Paleocene Deccan in

India (Figure 10.12). The Columbia River Plateau of

the northwestern United States (Figure 10.13) consists

of more than 100 ﬂows emplaced in a remarkably brief

period of the Miocene—almost entirely 17–15 Ma

(Reidel and Hooper, 1989). Their aggregate volume is

about 180,000 km

, covering an area of 160,000 km

a depth as much as 3 km. By comparison, the large

Mauna Loa shield volcano on the island of Hawaii has

one-sixth this volume.

Individual large lava ﬂows, literally ﬂoods, whose

stratigraphic correlation has been facilitated by “chem-

ical ﬁngerprinting” using particular element concentra-

tions and ratios, range in volume from 90 km

to per-

haps as much as 3000 km

. Some ﬂows traveled

hundreds of kilometers down gentle slopes of 1 m in

10 km (1/10,000). On the basis of a model of lava

transport in inﬂated pahoehoe ﬂows, Self et al. (1997)

propose that the gigantic compound Roza ﬂow ﬁeld

(Figure 10.14) was emplaced over a period of 6–14 y

and individual ﬂows in 5–50 months. Lava was able to

travel great distances because it did so under an insu-

lating pahoehoe crust and the only place where the

hottest mobile lava became exposed was at local break-

outs feeding new pahoehoe tongues. Calculated effu-

sion rates are about 4000 m

/s, which is comparable to

that of the 1783 Laki, Iceland, ﬁssure eruption cited

previously. Though such discharge rates seem extraor-

dinary, the rate for the peak 2-My plateau-forming

episode is about 0.08 km

/y, comparable to discharge

rates of Hawaiian shield volcanoes and oceanic rifts.

In ﬂood basalt plateaus, geologically rapid with-

drawal of 10

10

of magma did not result in

caldera collapse, as in silicic ash-ﬂow eruptions of com-

parable volume (discussed later). Roofs over shallow

crustal silicic chambers tend to be thinner than their

spanning diameter and hence readily collapse. In con-

trast, roofs over magma reservoirs feeding basalt ﬂoods

that lie in the upper mantle, possibly the lower crust,

have sufﬁcient thickness and strength to resist collapse

and merely subside over time. Discharge of huge vol-

umes of compositionally relatively uniform basalt

magma from the mantle raises questions regarding

magma generation and storage.

10.2.4 Submarine Basaltic Accumulations

Most submarine extrusions are of basaltic lava and ac-

count for most of global volcanism (Figure 1.1). Most

submarine activity involves ﬁssure eruptions at oceanic

ridges, whereas more localized central eruptions have

built in excess of one million basaltic volcanoes dotting

the seaﬂoor (Plate I). Most of these volcanoes are sub-

252 Igneous and Metamorphic Petrology

10.12 Plateau ﬂood basalts exposed in the Western Ghat escarp-

ment of the Deccan Plateau, India. Photograph provided

courtesy of Peter R. Hooper. [Reproduced by permission from

Hooper PR. Flood basalt provinces. In Encyclopedia of Vol-

canoes, Sigurdsson H, Houghton B, McNutt SR, Rymer H,

Press (a division of Harcourt Brace and Company).]

0 km 150

0 miles 100

Oregon

45°

Idaho

48°N

118°122°W

Washington

Pacific Ocean

10.13 Columbia River plateau ﬂood basalts, northwestern United

States. Subparallel north-northwest-trending lines show the

approximate location and orientation of known groups of

feeder dikes that number in the thousands. Extent of Roza

ﬂows (Figure 10.14) indicated by dashed line. (Redrawn from

Reidel and Hooper, 1989.)

merged seamounts 50100 m high; others have

grown from a submarine base (Figure 10.11c) into sub-

aerially exposed volcanic islands such as Hawaii.

The conﬁning pressure exerted by seawater and es-

pecially the relatively low volatile concentrations in

basalt magmas preclude explosive eruptions in all but

the shallowest water depths. Because hydrostatic pres-

sure increases by about 1 bar ( 10

Pa) for every 10 m

of water depth and because oceanic ridge basalt mag-

mas typically have small water concentrations of

0.5 wt.%, exsolution is limited to water depths of

500 m (Figure 4.7). Ocean ridge basalt magmas may

contain concentrations of CO

comparable to that of

water but, because of its much lower solubility (Figure

4.10), may exsolve below the seaﬂoor. Moreover, ex-

plosive fragmentation of magma due to bubble growth

cannot take place until some critical bubble volume

fraction develops, and this requires shallower depth for

bubble expansion. After a volcano has grown from the

deep seaﬂoor to shallower depths of perhaps 200 m,

explosive fragmental deposits can be formed.

Magmatism along the 65,000-km-long system of

oceanic spreading ridges around the world (Fron-

tispiece and Plate I) is the most proliﬁc on Earth (Fig-

ure 1.1). In concert with seaﬂoor spreading, the en-

tire present-day oceanic crust has been produced in

200 My

Extrusions along oceanic ridges at water depths of a

few kilometers typically produce pillow lava and,

Magma Extrusion: Field Relations of Volcanic Rock Bodies

253

(a)

Washington

100

miles

Oregon

Idaho

(b)

(c)

(d)

10.14 Schematic development of the Roza compound pahoehoe ﬂow

ﬁeld in the Columbia River plateau (Figure 10.13). Arrows in-

dicate direction of travel of basalt lava from ﬁssure vent source

(heavy line) beneath the insulating pahoehoe crust. (Redrawn

from Self et al., 1997; Martin, 1989.)

Special Interest Box 10.1 Speculative trigger-

ing of El Niño by submarine basalt extrusions

In the last two decades of the 1900s, a major

anomaly in the weather pattern in some large conti-

nental areas surrounding the Paciﬁc Ocean gained

considerable attention. This so-called El Niño is

caused by episodic perturbations at irregular recur-

rence intervals among sea surface temperatures, sea

level anomalies, and atmospheric wind stress. Shaw

and Moore (1988) have proposed a novel but con-

troversial hypothesis that El Niños might be trig-

gered by extrusion of large-volume submarine

basalt ﬂows along the East Paciﬁc Rise off the

northwest coast of South America. This is the fastest

oceanic spreading ridge in the world and is the area

where El Niños seem to be spawned. Shaw and

Moore speculate that some of these submarine ﬂows

may have volumes and extrusion rates comparable

to those of the ﬂood basalt lavas that formed conti-

nental plateaus. Every 1 km

of submarine basalt

that cools to ambient ocean temperatures within the

time of an El Niño cycle, they calculate, accounts

for about 1% of the anomaly.

where extrusion rates are greater, sheet lava ﬂows, lo-

cally pahoehoe. Along the Mid-Atlantic Ridge, a

crestal rift valley, 20 to 30 km wide and 2 km deep, has

extensional fault-controlled terraces ﬂanking a narrow

inner valley only a few kilometers wide and 100 to

400 m deep. Topographically controlled lava ﬂows

on the inner valley ﬂoor are elongate (Figure 10.15).

Along the faster-spreading East Paciﬁc Rise, lava lakes

have ponded on low-relief ﬂanks. One youthful ﬂow

discovered by side-looking sonar covers 220 km

and

has a volume estimated at 15 km

(Shaw and Moore,

1988).

Counterparts of the huge continental ﬂood-basalt

plateaus were discovered during investigations of the

ocean ﬂoors in the late decades of the twentieth cen-

tury (Mahoney and Cofﬁn, 1997). Oceanic plateaus are

broad topographic highs rising 1 km or so above the

surrounding seaﬂoor and underlain by a crust as much

as 40 km thick, ﬁve to six times typical oceanic crust.

Little is known of these plateaus.

10.3 EFFUSIONS OF SILICIC LAVA

10.3.1 Morphological Characteristics and Growth

The most crystalline and, especially, the most silica-rich

lavas are the least mobile because of their high appar-

ent viscosities. Accordingly, silicic effusions have much

larger aspect ratios (thickness/length) than typically

sheet-like basaltic lava ﬂows. A spectrum of shapes can

254 Igneous and Metamorphic Petrology

10.15 Evolution of the inner rift valley in the northern Mid-Atlantic Ridge at 36 50N latitude during the past 0.2 My. Shaded areas show

volcanic ediﬁces built along the active volcanic axis (VA) that are later cut by a subsequent rift (SR). Lower right diagram combines

all of these rifts. Small numbers give ages of volcanic ediﬁces in thousands of years. (From Ballard RD and van Andel TH, Morphology

and tectonics of the inner rift valley at lat 36 50N on the Mid-Atlantic Ridge. Geol. Soc. Am. Bull. 88:507–530, 1977; Reproduced

America.)

be recognized (Figure 10.16). The most mobile with

the smallest aspect ratio is the lava ﬂow (Figure 10.17).

The more viscous mushroom-shaped lava dome (Fig-

ures 10.2, 10.5, 10.18) has a larger aspect ratio. The

still larger-aspect-ratio peléan dome resembles an arti-

choke and grows by expansion from within, pushing

slabs of rigid lava out of and away from the vent in fan

fashion or along sled-runner-shaped ramps. Although

most silicic effusions have blocky fractured tops and

surrounding aprons of blocky talus (Figure 7.33), these

are more pronounced in peléan domes. Slender spines

may be elevated above the remainder of the peléan

dome before being shattered by steam explosions or

thermal stresses or collapsing under their own weight.

Least mobile, rigid lava is extruded as an upheaved

plug, which is an elongate cylindrical mass roughly the

diameter of the vent conduit so that its aspect ratio is

1; a plug resembles a cork in a narrow-necked wine

bottle.

Many domes grow in craters (Figure 10.5) produced

by preceding explosive eruption of more volatile-

rich magma from the top of the supply chamber. Ex-

plosive activity may continue during the slow continu-

ous or episodic effusion of lava, which advances from

the vent at velocities of meters per hour to meters per

day. The duration of lava effusion can be many decades

in the case of large domes, such as the 1-km

growth

of Santiaguito, Guatemala, since 1922 (Rose, 1987).

In some effusions it is clear that new lava is slowly

added in an endogenous manner (Figure 10.19), inﬂat-

ing the interior beneath a more rigid carapace or cover

of closely packed blocks of lava. Other effusions are

exogenous and grow by addition of lava onto the

surface.

Dome growth may be modeled in two ways, as a

brittle shell enclosing pressurized magma or as vis-

coplastic lava. In the viscoplastic model (Blake, 1990;

see also Section 8.2.2), the chilled carapace of a low

lava dome has yield strengths of 10

10

Pa, whereas a

steeper-sided peléan dome, whose form is dictated as

much by its talus apron as by the rheology of the lava

core, has a yield strength 10

10

Pa.

Autoclastic fragments, generally of block size, man-

tle ﬂows and domes and form talus aprons around

them (Figure 7.33). Fragments are created by move-

ment of the rigid carapace, thermal stresses, or internal

gas pressure, or by collapse under their own weight.

As the effusion advances laterally, the clastic apron may

be pushed aside or overridden and possibly engulfed

within the ﬂow. Larger-volume crumbling of sectors of

the steep dome and plug margins on summits or on

slopes of larger volcanoes creates avalanches of consid-

erable runout, as in the Chaos Jumbles in Figure 10.18.

More explosive shattering generates devastating pyro-

Magma Extrusion: Field Relations of Volcanic Rock Bodies

255

Upheaved plug

100

1000

10,000

Radius, R (m)

Low lava dome

Idaho

lava flows

100

1000

Height, H (m)

0.55

H/R = 1.06

Pelean

dome

10.16 Shapes of silicic effusions. Cross sections through typical low lava dome and peléan dome, both resting on precursory pyroclastic de-

posits, shown as dotted lines. (Redrawn from Williams, 1932.) Note aprons of mostly block-size talus. Measured dome height, H, versus

dome radius, R, of selected low lava domes (ellipses) and peléan domes (triangles) around the world are plotted with a best-ﬁt line rep-

resenting the indicated aspect ratio, H/R. (Redrawn from Blake, 1990.) The aspect ratio of peléan domes is similar to the tangent of an-

gles of repose of the talus pile of unconsolidated fragments that typically surround them. Note that these talus accumulations are signif-

icantly less around low lava domes. Shaded rectangle on right represents large-volume (as much as 200 km

), high-T rhyolite lava ﬂows

in southwestern Idaho. (From Bonnichsen and Kauffman, 1987.) Shaded band on left represents possible aspect ratios for high-yield-

strength upheaved plugs. As silicic lava emerges from a vent (arrow, lower left), with increasing height, H, it may grow into a lava ﬂow,

low lava dome, peléan dome, or upheaved plug depending on increasing yield strength.

clastic avalanches that cascade with hurricane speed

downslope.

10.3.2 Internal Fabric

Planar and linear fabric elements are widespread in sili-

cic effusions and constitute markers of the pattern of

internal ﬂow in the body during emplacement. More or

less planar ﬂow layering (Figures 7.4 and 7.41) ex-

pressed by textural variations (crystal size, crystallinity,

vesicularity) in a wide range of lava viscosities reﬂects

laminar ﬂow. Contorted and folded ﬂow layers reﬂect

local changes in ﬂow velocity or drag, particularly near

ﬂow margins. Grooves and striations, which resemble

slickenlines on fault surfaces, and associated transverse

open tension gashes also develop on shear surfaces in

viscous lava.

Rhyolite ﬂows and domes have a more or less con-

sistent vertical internal zonation of fabric developed by

variable vesiculation, fragmentation, and devitriﬁcation

superposed on rheologic ﬂow. This zonation serves as a

useful ﬁeld guide in the interpretation of poorly ex-

posed old ﬂows. Thicknesses of zones vary consider-

ably, and a particular zone may not be everywhere

present. Tops and bottoms of the extrusion are of au-

tobreccia that is variably vesiculated, as are local inter-

nal seams representing brecciated ﬂow margin material

engulfed during ﬂow or zones of rigid magma that were

fragmented. Interiors are ﬂow-layered. Young effusions

256 Igneous and Metamorphic Petrology

10.17 Vertical aerial photograph of Big Glass Mountain rhyolite obsidian ﬂow east of Mount Shasta, California. Main tongue toward upper left

is nearly 3 km long, has a thickness of about 75 m, and ﬂowed down an approximate 9 slope from the vent near photo center. Arcuate

pressure ridges in ﬂow surface transverse to ﬂow direction that are spaced about 60 m apart with amplitudes averaging 8 m possibly orig-

inated through compressional drag forces exerted in the more rigid ﬂow surface by the underlying hotter, more mobile ﬂowing interior.

Note east-northeast trending line of extrusive vents beginning at lower left corner and ending at upper right that were probably con-

trolled by a ﬁssure. (Photograph by U.S. Forest Service.)